1. Field of the Invention
The present invention relates to the devices semiconductor devices, such as a semiconductor laser chip, an endface incident type photodiode, or an endface incident-type semiconductor amplifier, which may be incorporated with high precision into an optical communication module, and methods of manufacturing the devices.
2. Description of the Related Art
Among the methods for incorporating a semiconductor laser chip into an optical communication module are an "active alignment method" and a "passive alignment method". In the active alignment method, the optical axis of a semiconductor laser chip, while emitting light, is aligned with an optical fiber, the semiconductor laser chip being fixed to a submount at a position where an optical output may be obtained.
In the passive alignment method, on the other hand, the semiconductor laser chip is positioned such that an alignment mark on the semiconductor laser chip is superimposed on an alignment mark on the submount, and the semiconductor laser chip is then fixed to a submount.
Since alignment of optical axes generally takes a long time, the passive alignment method not requiring alignment of optical axes results in a better throughput and allows the fabrication of an optical communication module at a lower cost as compared to the active alignment method where alignment of optical axes is required.
In the passive alignment method, however, the precision of the position of the alignment mark on the semiconductor laser chip with respect to a waveguide (emitting point) has a large effect on the coupling efficiency of an exiting laser beam to an optical fiber. Accordingly, an optical communication module having an excellent coupling efficiency cannot be achieved without improving the alignment mark positioning precision.
A known optical semiconductor device and passive alignment method are described with reference to FIGS. 26A to 27C. FIG. 26A is a plan view of a known semiconductor laser chip. FIG. 26B is a cross-sectional view of the known semiconductor laser chip taken along line 26B--26B of FIG. 26A. FIGS. 27A to 27C show a passive alignment method using a known semiconductor laser chip. The structures of FIGS. 26A to 27C are not known to the public but represent a created technology for explaining the invention.
Referring to FIGS. 26A and 26B, a known semiconductor laser chip 1 includes a substrate 2, an active layer 3 (optical waveguide 3a), a current blocking layer 6, a contact layer 7, an insulating film 8, a front surface electrode 9, a pair of alignment marks 10 formed at the same time as the front surface electrode 9, and a back surface electrode 11. The alignment marks 10 are the same material as the front surface electrode 9.
Referring to FIGS. 27A to 27C, a submount 20 includes a substrate 21, an optical fiber 22 on the substrate 21, a metal pattern 24 on the substrate 21 and having a pair of alignment marks 23, and a solder 25 on the metal pattern 24. The pair of alignment marks 23 are holes in the metal pattern 24 at symmetrical positions with respect to the center line of the optical fiber 22.
The known passive alignment method is now described. The semiconductor laser chip 1 of FIG. 27A, while being aligned by means of infrared light, is die bonded, as shown in FIG. 27C, to the submount 20 of FIG. 27B. As a coarse alignment step, the semiconductor laser chip 1 is placed on the submount 20, for example, with a vacuum tweezer. The coarse alignment uses pattern recognition with the metal pattern 24 indicating the mounting position of the semiconductor laser chip 1 on the submount 20, and infrared light transmitted through the front surface electrode 9 or back surface electrode 11 of the semiconductor laser chip 1. For example, infrared light incident on and transmitted through the semiconductor laser chip 1 is detected by a CCD (charge coupled device) disposed at the back surface of the submount 20.
The alignment mark 10 of the semiconductor laser chip 1, which does not transmit infrared light, and the alignment mark 23 on the submount 20, which transmits infrared light, are seen as overlapping, as shown in FIG. 27C, when infrared light is transmitted. Alignment is effected during transmission of the infrared light so that the respective centers of areas of the alignment marks 10 and 23 coincide. The metal pattern 24 and the back surface electrode 11 are then bonded to each other using the solder 25. Thereafter, an optical communication module is fabricated by mounting on the submount 20 an electrode for driving the semiconductor laser chip 1, a photodiode for monitoring an output laser beam, etc.
In the known semiconductor laser chip, the optical waveguide 3a and the alignment mark 10 are formed in different processes, i.e., the alignment mark 10 is formed together with the front surface electrode 9 in a processing step that is later than the processing step for forming the active layer 3. There has thus been a problem, reflecting a limit of precision in superposition of a mask aligner, that an offset B as shown in FIG. 26A necessarily occurs between the center line of the optical waveguide 3a and the center line (bisector) of the pair of alignment marks 10.
Further, since this offset B is usually on the order of several microns, it is very difficult to achieve a precision on the order of submicrons, as required, using the passive alignment method. If, as shown in FIG. 27C, the semiconductor laser chip 1 with the offset B is used, the coupling efficiency between the optical waveguide 3a and the optical fiber 22 is poor. It has thus been difficult to obtain an optical communication module having an excellent coupling efficiency at a high yield.